JUBA - War-torn South Sudan faces possible famine early next year, the UN chief in the country warned Thursday, as aid workers said the shooting down of a UN helicopter threatened efforts to save lives.

"We all are working very hard to prevent a famine... but I am very worried that we will not be able to prevent it," UN aid chief in South Sudan, Toby Lanzer, told reporters.

Thousands of people have been killed and more than 1.8 million have fled a civil war sparked by a power struggle between President Salva Kiir and his sacked deputy Riek Machar.

"The single biggest cause if there is a famine is the failure of the political leadership to resolve this crisis," Lanzer said.

Famine, if declared, could be expected at the end of 2014 or "more likely" in early 2015, he added.

The UN Security Council said the UN Mi-8 cargo helicopter was downed Tuesday by an "attack" that killed three Russian crew members and injured another, but did not say which side was to blame.

The UN says it is worried it may not be able to prevent the looming famine in South Sudan. (AFP)

Lanzer said that all flights to the northern oil town of Bentiu, had been suspended following the crash, as investigators examined the craft's black box flight recorder.

"If this type of threat continues, our services will grind to a halt in Bentiu," Wendy Taeuber, who heads International Rescue Committee (IRC) in the country, told AFP.

"Helicopter is the only way in and out for both staff and supplies."

The army and rebels have both accused each other for the attack, which broke a day-old ceasefire deal, the fourth in eight months of war.

Over 45,000 civilians are sheltering in the UN camp in Bentiu alone, some of more than 100,000 civilians who have fled to UN bases to escape the conflict.

Famine implies that at least 20 percent of households face extreme food shortages, there is acute malnutrition in over 30 percent of people, and two deaths per 10,000 people every day, according to the UN's definition.

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